1. Field of the Invention
The present invention relates to a chamfering apparatus for chamfering glass substrates used for recording media disks of hard disk drives or wafers for semiconductor devices.
2. Description of the Related Art
Information terminal devices, such as cell phones and portable music players, equipped with a high-capacity hard disk are now available. There has been a requirement to increase the capacity and reduce the size of a magnetic disk that is a recording medium used in a hard disk drive. To fulfill such a requirement, these days a glass substrate is used as a substrate of a magnetic disk used in a hard disk drive instead of a conventional aluminum alloy substrate.
The glass substrate is machined into a disk shape having a circular hole at the center. However, inner and outer peripheral edges of the glass substrate are prone to be cracked due to an internal stress. Therefore, chamfering is performed on the inner and outer peripheral edges of the glass substrate. Chamfering is performed on a peripheral edge of even a single crystal silicon wafer for a semiconductor device to suppress cracking, chipping, or a breakage during a delivery process or a device process. For example, Japanese Patent Application Laid-open No. 2003-231044 discloses a chamfering technology for chamfering a glass substrate.
In the Japanese Patent Application Laid-open No. 2003-231044, as shown in FIG. 13, chamfering is performed in such a manner that outer peripheries of cylindrical rotary grindstones 110 and 111 are pressed toward an outer end surface 101 and an inner end surface 102 of a disk-shaped glass substrate 100 while the glass substrate 100 is rotated. As shown in FIG. 14, a plurality of grinding grooves 112 is formed along the entire circumferences of the rotary grindstones 110 and 111. Each of the grinding grooves 112 tapers toward a groove bottom. Moreover, grinding grain, such as diamond grain, is adhered on the inner surface of the grinding groove 112. A tapered surface 112a of the grinding groove 112 contacts with the outer end surface 101 or the inner end surface 102 of the glass substrate 100, so that the inner end surface and the outer end surface are ground because of friction and the edge portions are chamfered. Japanese Patent Application Laid-open No. H9-181021 also discloses beveling of a wafer by employing a similar chamfering method to the above.
However, as shown in FIG. 14, at a portion “a” that is in contact with an edge of the glass substrate 100, scarring (liner concavity) is formed on the periphery of the tapered surface 112a. When such scarring is formed, replacement of the grinding groove 112 used for grinding the glass substrate 100 is required (for example, the grinding groove 112 on a lower stage is used). Alternatively, dressing of the rotary grindstones 110 and 111 is needed. Thus, management of grindstones is complicated in the conventional grinding device. Furthermore, replacing the grinding groove 112 may change grinding conditions, and vary grinding finishes. Because the glass substrate 100 contacts with the outer peripheries of the rotary grindstones 110 and 111 always on the same trajectory, chipping may easily occur in a region ranging from the chamfered portion to the surface of the glass substrate 100. Moreover, strip scarring may be formed easily on the grinded surface. Particularly, because high precision machining is required for glass substrates used as information recording media, finishing defects at a boundary of the grinded surface and the glass substrate surface, and finishing defects at the grinded surface can cause trouble.